The Evolutionary Purpose of Male Homosexuality

A recent show on the science channel alerted me to an interesting new hypothesis about the evolution of homosexuality. Contrary to popular thought, it does not appear to be an accident. (If anyone can find documentation about this new hypothesis... let me know). Particularly interesting is the behavior of primate males in family groups. As is well known, apes (such as chimanzees, bonobos, and gorillas) that live in family groups consist of many females, one dominant male that gets exclusive access to mating with the females, and sometimes one or more submissive males that do not have the right to mate with the females. A new scientific hypothesis suggests that our early ancestors (who may have lived in similar family groups) may have developed homosexuality among these "submissive" males. The sexual behavior of the apes currently being studied suggests that male homosexuality has an important evolutionary advantage in family groups. The submissive males engage in homosexual behavior as a way to satisfy sexual instincts... but the advantage goes further than that. By engaging in homosexuality, the submissive males present themselves as no threat to the dominate male and thus promote group harmony, by discouraging male fighting over mates. Furthermore, these homosexual males provided survival advantages to the family group by helping to protect the females and infants from predators and rival family groups and to find food for the group. In this case, the sacrifice of the genetic survival of one individual gave an enormous advantage to the survival of the group. - Which is something evolution has been known to favor.

Although this particular advantage of homosexuality is obsolete for humans, the homosexual orientation would have survived despite that because our society changes far faster than evolution.

So.. for any homophobes that claim homosexuality is "unnatural" - Au Contraire! It is VERY natural, for some people!

Look! Guys! I used "submissive" not in the sexual sense but in the hierarchy of primate family groups. There is much more research to be done and this is simply a hypothesis that could be an interesting research opportunity. The whole idea that "submissive" is a negative trait is part of human prejudice anyway. Centuries of looking at stereotypically "feminine" personality traits as "weak" and "undesirable" has had a devastating effect on the rights of both gays AND women. Everyone is oppressed when a gender roles straight jacket is applied. Nature doesn't always have a male in charge anyway! Remember elephants? Who's in charge of those herds? Females! Submissive and or "feminine" or "nuturing" or "gentle" aren't bad traits. This is all part of nature. People need to learn that! Relax!

Recent evidence from embryology suggests that sexual orientation AND identity are developed early in gestation. Basically, every embryo starts out "female." When the genes determining sex kick in they appear to develop not only physical sexual organs but also sexuality in the brain. No matter what your sexual identity or orientation... you REALLY ARE born that way.

Homosexual apes simply don't want what the alpha male is trying to protect and so wont challenge him for it.

That's the key point of the argument. That over time, in family groups where only one male gets access to females, evolution would favor those males who do not pose a threat to the alpha male because they aren't interested in the females.

@skycomet thank you, that helped me understand. There must be a balance between aggression and submission, and maybe we'll learn someday how those dynamics are determined and how they play out in a species.

I tend to believe that aggression (whether or not related to male testosterone) was like fuel on a fire during our early cultural evolution, for better or worse. I.e., for better or worse, it made cultural evolution happen much faster than it otherwise would have, and now it's time for us to temper our aggression... not so much with submission, but with a more libertarian "don't force one's ways on others" idealism. (And I say idealism with full knowledge of how flawed any idealism can be. I.e., we still need new wisdom from philosophy and experience to hash out some ideas.)

Anyway, if nothing else, it's at least important for us to learn and understand the differences between natural/genetic evolution, and artificial/cultural evolution.

It would be a mistake to automatically assume that non-human primate one-male-several-female groups are dominated by males. Among geladas, females largely control the social group. This is despite the fact that the males are larger, stronger, and more aggressive. Mothers, sisters, and aunts act as a team in chasing off other unrelated females. They also collectively select their mutual mate among a number of potential suitors roaming in and out of their territory. The male that is chosen usually is one that does not act abusively towards them and is willing to cooperate with them in defending their territory. The relationship with any particular male may be short-term. The stable core of the community is the group of related females. This is a long way from stereotypical male domination.

One-male-several female groups may take a different form when predator pressure is a problem. In open grasslands, hamadryas baboon communities are much larger, usually consisting of a number of polygynous families. In such multiple one-male-several-female group societies, males are the dominant, controlling members. The adult males not only "herd" their own sexually mature females, but also maintain order and protect the community from predators. This is not unlike the traditional Arab polygynous marriage pattern in which wealthy men acquire harems.

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Male chimps generally prefer to mate with females who are 30 years of age or older. These older females are also more often fought over by males. Even balding 55 year old females with worn and broken teeth apparently are more appealing to males than are young healthy females, aged 15-20. Muller speculates that the appeal of older females is that they have been successful in surviving and in having and raising offspring.

Interesting, no? Hope that helps. Er, without derailing this thread! But what I infer from what I've read (and what's most interesting) is that the dominant male and/or mating femail who pass on such dominance/aggressive genes will also likely be passing on traits (via genes, chromatin, epigenetics, and maybe even culture) that make a large percentage of the males submissive to the hierarchy. I.e., perhaps what's passed on to offspring is not aggressiveness as a single trait, but a wide range of variability of aggressiveness among the same family's offspring.

@arch - not necessarily. Evolution will occassionally favor the sacrifice of an individual's reproduction if it means that this provides a significant advantage for the survival of the species as a whole.

according to Mendel's Laws of Heredity - there be a 25/75 ratio of gays to straights? (I use the two terms because they take up less space to type)

Setting homosexuality aside for a moment, what you wrote is grossly inadequate when discussing genetics. There are more variations that could conceivably be at play than I care to list, some of which are covered in entry level genetics units (such as multi-allelic traits, polygenic traits, and traits with multiple phenotypes). While I'm not suggesting that homosexuality fits any of those categories (if it even is genetic in part or in while), when there are som any unknowns, conversations on genetics cannot be limited to such simple inheritance.

@arch: It seems to be a mixture between biological predispositions and environmental factors. There was a fairly widely reported and comprehensive Swedish twin study which has now been added to Wiki:

A 2010 study of all adult twins in Sweden (more than 7,600 twins) found that same-sex behavior was explained by both heritable factors and individual-specific environmental sources (such as prenatal environment, experience with illness and trauma, as well as peer groups, and sexual experiences), while influences of shared-environment variables such as familial environment and societal attitudes had a weaker, but significant effect. Women showed a statistically non-significant trend to weaker influence of hereditary effects, while men showed no effect of shared environmental effects. The use of all adult twins in Sweden was designed to address the criticism of volunteer studies, in which a potential bias towards participation by gay twin may influence the results (see below).

@arch: By me? In that case I have not made myself entirely clear, as my opinion is that it is a mixture, though with the environment as the main factor. I usually get blasted since that position apparently implies that it would a conscious choice. A choice perhaps, but not conscious.

I think this is flawed because it wouldn't correlate to female homosexuality. Also if it was true then all males who failed the mating game would turn homosexual, not just a few. If some submissive males can remain hetero, then all males would stay hetero because evolution only makes changes that are necessary to survival. If it isn't necessary for all submissive males to become homosexual then why would it be necessary for a select few to become homosexual?